Heat exchangers of a shell and coil design have been used for many years in a variety of applications where it is desired to transfer energy between two fluids. Shell and coil heat exchangers are frequently used in refrigeration systems and heat pumps. Shell and coil heat exchangers can be fabricated into a compact unit capable of withstanding relatively high pressure.
Shell and coil heat exchangers are typically mounted vertically, i.e., the axis about which the coil is wound is perpendicular to the ground. With a vertical shell when you have a gas vapor mixture, the gas will tend to accumulate at the top with the shell and the liquid will accumulate at the bottom. The flow of the fluid in the shell is generally axial flowing from one end to the other and circulating about the coils of tubing within the shell cavity.
In order to minimize the volume within the shell a central tubular insert may be provided which falls within the helical coil. This is particularly useful in refrigeration systems and heat pumps so that the quantity of refrigerant may be minimized. Example of a shell and coil heat exchanger having an inner shell to minimize shell cavity volume is shown in U.S. Pat. No. 2,668,692 and companion U.S. Pat. No. 2,668,420. In spite of the inner shell, a significant disadvantage of the shell heat exchangers are the large volume of the shell cavity relative to the volume of the liquid within the coiled tubing.